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Exposures to polychlorinated biphenyls (PCB) and chlorinated benzenes were surveyed on June 23 and 24, and July 14 to 20, 1982 at Hill Murray High School (SIC-9224, SIC-4911) in Maplewood, Minnesota. The evaluation was requested by the Minnesota Pollution Control Agency and the Minnesota Department of Health on behalf of 61 individuals present when a power transformer overheated and vented 50 gallons of dielectric fluid into the transformer vault. Surface and air samples were collected, and medical questionnaires and blood and urine tests were given to the potentially exposed individuals. Within 40 hours of the incident airborne PCB concentrations ranged from 50 to 90 micrograms per cubic meter (microg/m3) inside the vaults, and measured 20microg/m3 immediately outside the vault. The NIOSH recommended exposure limit is 1microg/m3. Trichlorobenzene (12002481) and tetrachlorobenzene (12408105) concentrations did not exceed available recommended limits. Decontamination procedures effectively reduced surface PCB concentrations. The exposed individuals reported symptoms such as itching, skin irritation, fatigue, and headache during the week after the incident occurred. No increases in blood PCB values were found, and urine chemistry was normal except for a transient, subacute elevation of some hepatic enzymes. The authors conclude that a potential acute exposure hazard occurred at the time of the transformer overheating incident. They recommend institution of five hazard inspection and preventive maintenance programs.